LINCOLN, Neb. 
Sen. Ben Nelson of Nebraska said Tuesday he will retire rather than seek a third term next year, dealing a significant setback to Democratic efforts to maintain control of the chamber.

The 70-year-old conservative Democrat, whose seat is being heavily targeted by Republicans in 2012, said in a statement that "while I relish the opportunity to undertake the work that lies ahead, I also feel it's time for me to step away from elective office, spend more time with my family, and look for new ways to serve our state and nation."

"Therefore, I am announcing today that I will not seek re-election. Simply put: It is time to move on," he said.

Republicans, who need to net four seats to take back the Senate in next year, say Nebraska has tilted further to the right in recent years. Nelson is the lone Democrat among the state's five-member congressional delegation, and the Republicans think Nelson's vote for President Barack Obama's signature health care legislation would have weighed him down.

Nelson still could have given Democrats a fighting chance. A two-term governor before winning a Senate seat, he has shown an ability to rebound after being down in previous statewide races.

But he's recently expressed dismay about a divided Congress' inability to pass meaningful legislation, frustration that echoed in the Tuesday statement in which he said public office is "about promoting the common good, not the agenda of the radical right or the radical left."

"I encourage those who will follow in my footsteps to look for common ground and to work together in bipartisan ways to do what's best for the country, not just one political party," he said.

Even as Nelson wavered about whether to seek another term, he piled up campaign cash, hired a campaign manager and watched his party spend more than $1 million on ads supporting him. Prominent Nebraska Democrats said they were shocked by his Tuesday announcement and said they were concerned about who they might field against the Republican nominee.

"I'm absolutely stunned," said Kathleen Fahey, a Democratic super-delegate in 2008. "Ben has been such a great senator for everybody. I'm not liking this."

The campaign preparation had left him with a healthy cash advantage. He had more than $3 million cash on hand last month, about twice his nearest competitor, and had the luxury of stockpiling money while Republicans focused on a crowded primary that includes Don Stenberg, the state's treasurer, Jon Bruning, the attorney general, Deb Fischer, a state senator, and Pat Flynn, an investment adviser.

"In one respect, he certainly deserves to have some time away from the political scene," said Steve Achelpohl, a Democratic super-delegate. "But on the other hand, it certainly is going to leave a void."

Nelson was first elected to the Senate in 2000, defeating Republican contender Stenberg to replace the retired Bob Kerrey. His centrist stance helped him get re-elected over the years even as already-conservative Nebraska became even more dominated by Republicans.

He was one of only two Senate Democrats to side with Republicans earlier this year on a failed GOP bid to block new federal controls on power plant pollution that blows downwind into other states. And he took great pride in his membership in the 2005 "Gang of 14," made up of Republicans and Democrats who brokered a deal to avoid a filibuster showdown over President George W. Bush's judicial nominees.